A conventional electric discharge machine includes a chuck device as shown in FIGS. 5 to 7 and generally includes a connector 7 connected with a discharge head (not shown) and a mold 60 which is fixed to the machine. The connector 50 includes a plurality of protrusions 52 extending along a periphery of a top thereof and a connection rod 51 is located at a center of the top of the connector 50. The connection rod 51 includes a neck 511 and each of the protrusions 52 includes a substantially U-shaped recess 53 defined in a top thereof. Each recess 53 includes two inclined surfaces 531 which face each other. The mold 60 has a central hole 61 with which the connection rod 51 is engaged. A plurality of positioning circular bars 62 are fixed to an underside of the mold 60 by bolts and engaged with the recesses 53.
As shown in FIG. 7, the positioning circular bars 62 are in contact with the two inclined surfaces 531 such that the connector 50 and the mold 60 do not have any relative rotational movement. During discharging processes, the object to be machined applies a reaction force to the discharge head and the neck 511 of the connection rod 51 is broken if the reaction force is too large. The positioning circular bars 60 then slide along the inclined surfaces 531 to remove from the recesses 53 so as to protect the mold 60 from being damaged. However, the contact areas between the positioning circular bars 62 and the inclined surfaces 531 are so limited so that the contact areas cannot bear too much stress, the connector 50 and the mold 60 might be separated by a force over a certain level and this usually causes delay to the procedures.
Another conventional chuck device is disclosed in FIGS. 8 to 10, wherein the connector 70 includes several recesses 72 defined in a top 71 thereof and a connection rod 701 extends from the top 71. A plurality of L-shaped plates 73 are connected to protrusions on the top 71 of the connector 70 and each plate 73 includes two openings 731 which face the recesses 72 and have the same width as the recesses 72. A gap 74 is defined between the top 71 and the plate 73. The mold 80 includes blocks 81 extending from an underside thereof and the blocks 81 are inserted in the recesses 72 and in contact with peripheries of the openings 731 as shown in FIGS. 9 and 10.
In the connection between the connector 70 and the mold 80, the blocks 81 are in contact with the vertical peripheries of the openings 731 so that the contact area is very limited. When the neck 702 of the connection rod 701 is broken, the limited area cannot share too much force for the neck 702.
The present invention intends to provide a chuck device for electrical discharge machine wherein the contact areas between the positioning bars on the mold and the recesses in the connector and for engaging the positioning bars are large enough so that the neck of the connection rod is not broken easily.